There is a way to delay larvae from pupating, this I saw on tv where somebody made his living breeding
maggots. when they were large enough and close to pupation, the maggot-farmer put the maggots in boxes and
placed all boxes in a cooling cell. I don't think we'll have a cooling cell in the aftertime though. Well, maybe 2%
of those preparing will have to some degree the technology to have one. So we have to be quickly eat the critters
before they pupate, which is a matter of timing.
In the case of larvae of beetles I think the beetles can be eaten, too. And should be left to pupate here and there so they can mate and provide new larvae. In order to prevent escape a simple plastic aquarium (they're almost unbreakable, have rounded corners etc.) could be used, over which one fits the top with a lid with gaze/grate in it. Of course, opening it will give some the chance to fly off (some beetles do fly).
Another solution is to make a hole in the side (burn it out with hot iron or something that you can hold in a fire). The plastic will melt and you can cut any shape from the tank). Make the hole at such a size that you can put your arm through it and touch the bottom of the tank. Now take a longstocking, glue it or otherwise firmly attach it to the outside of the hole. One should be able to put one's arm through the stocking, into the tank and take out what one wants. This way no critter will fly off. If you're done, bind a wire around the stocking so it is closed, so even beetles smart enough to reach the stocking can't get out. One can also do this with other critters that fly, like flies, locusts, other types of beetles, crickets. Of course, some beetles, locusts and crickets will be able to knaw/eat their way through the stocking, so a stronger material might be advised.
Offered by Michel.